This invention relates to testing fibre optic communication systems, particularly to devices for emulating optical fibre so as to test systems for communication over a relatively long section of optical fibre.
As communication protocols and advances in data transport technologies allow optical fibre connections to operate at increased distances, it becomes necessary to test fibre optic communication systems over increasingly longer sections of optical fibre. The use of a long section of optical fibre, for example, a fibre that is a kilometer in length, in a laboratory or production test facility is both expensive and physically inconvenient because of the size of the fibre spool required. In addition, in testing a fibre optic communications system or device, it is often desirable to perform tests over various lengths of fibre. This requires the use of multiple sections of optical fibre of different respective lengths, for example, one kilometer, 5 kilometers, and 10 kilometers, which is even more expensive and inconvenient. Therefore, it would be advantageous to be able to emulate sections of optical fibre of selectable length in a physically convenient and relatively inexpensive way.
The two primary characteristics of a section of optical fibre that must be emulated to test fire optic communications systems are signal attenuation and signal propagation delay. Devices have been developed for introducing variable attenuation in an optical fibre link. Such devices are disclosed, for example, in Stankos et al., U.S. Pat. No. 4,261,640 and Tamulevich U.S. Pat. No. 4,989,938. However, neither of these devices addresses the issue of signal delay.
The simulation of signal delay, as well as attenuation, in an optical fibre has been addressed by the disclosure of Deloddere et al. U.S. Pat. No. 5,777,765. In the device of Deloddere et al. an optical signal is demodulated to produce an electrical signal whose amplitude is representative of the intensity envelope of the optical signal. That electrical signal is then sampled periodically and the samples are digitized. The digitized samples are then applied to the input of a shift register, which propagates the samples at substantially the same as the original optical signal would propagate on an optical fibre, However, since the electrical fluctuations between samples can be ignored, provided that the sample frequency is twice the highest significant frequency component of the electrical signal, the information in the optical signal that would propagate a given distance in a given amount of time over an optical fibre can be compressed into a shorter distance for the same amount of time in the shift register. The shift register is tapped at a point chosen for the amount of signal delay it introduces, the output modulates an optical carrier, and an attenuator is applied to the modulated carrier to simulate an amount of attenuation corresponding to the position of the shift register tap.
However, the approach to simulation of delay in Deloddere et al. is not suitable for all emulation of an optical fibre to test optical fibre communication systems. Assuming that information is presented by a fibre optic channel as asynchronous digital, or binary, data, the sample rate to reproduce the data faithfully would need to be at least twice the information transfer rate in bits per unit time. Typically, in the absence of some synchronizing mechanism, a serial digital signal would be sampled at sixteen times the data rate. This allows the data pattern to be determined, but additional logic functions would be needed to interpret the data and to reclock the data before it is retransmitted to prevent excess jitter. Alternatively, the signal may be sampled at a frequency that is higher than the jitter tolerance of the system and the shift register would have to run at that same frequency. This is impractical with current technology.
Accordingly, there has been a need for a an improved method and device for emulating an optical fibre so as to render unnecessary the use of bulky and expensive spools of test fibre in laboratory and production testing of fibre optic communication systems.
The aforementioned need has been met for digital optical communications systems by the present invention by taking advantage of the digitally-encoded nature of information transmitted over fibre optic communications systems. The invention provides a device that receives a digitally-encoded optical signal; demodulates the optical signal to produce serial electrical pulses representative of the digital signal; converts the serial pulses to parallel pulses; and propagates the parallel pulses through a shift register whose propagation time is less than or equal to the propagation time of a section of fibre to be emulated. The device then converts the parallel pulses at the output of the shift register to serial pulses; modulates an optical carrier with the serial pulses from the shift register; and attenuates the modulated optical signal an amount corresponding to that of the fibre being emulated to produce an output optical signal whose propagation time is substantially equal to that of the section of fibre to be emulated. The shift register may be tapped at any point to obtain an output delayed by a desired amount corresponding to a selected section of optical fibre. By propagating digital information code, rather than digitized signal amplitude samples, the required signal delay corresponding to a given length of optical fibre can be achieved with a shift register of practical length and cost.
Accordingly, it is a principal object of the present invention to provide a novel and improved device and method for emulating optical fibre.
It is another object of the present invention to provide a device and method for emulating a long section of optical fiber in a physically practical way.
It is a further object of the present invention to provide a device and method for emulating a long section of optical fiber by taking advantage of the digitally-encoded nature of information transmitted over fibre optic communications systems.
The foregoing and other objects, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.